1. Field of the Invention
The present invention is related to the field of magnetic tunnel junction (MTJ) devices with particular reference to the magnetic random access memory (MRAM) and the magnetic sensors including the recording read head in hard disk drive and so on, which employ tunneling magnetoresistance. More particularly, this invention relates to the MTJ devices with MgO tunnel barrier prepared by oxidation methods or reactive sputtering method, microstructure of which is amorphous or microcrystalline with poor (001) out-of-plane texture. More particularly, this invention relates to the MTJ devices with the insertion of the crystalline ferromagnetic layers, which is the PGGP seed layers, adjacent to the MgO tunnel barrier in order to enhance the crystallinity of the MgO tunnel barrier during post-deposition annealing.
2. Related Arts
Core element in the magnetic tunnel junction (MTJ) device is “ferromagnetic layer/tunnel barrier/ferromagnetic layer” tri-layer structure. The change of resistance of the MTJ device is attributed to the difference in the tunneling probability of the spin polarized electrons through the tunnel barrier on the bias voltage across the device in accordance with the relative orientation of magnetizations of the two ferromagnetic layers.
The relative orientation of the magnetizations of the two ferromagnetic layers sandwiching the tunnel barrier is realized by the different nature of the magnetization reversal of the two ferromagnetic layers, in that the magnetization of one ferromagnetic layer is not reversed by the external magnetic field in operation, whereas that of the other ferromagnetic layer responds to the external magnetic field. Thus parallel or antiparallel alignment of the magnetizations of the two ferromagnetic layers sandwiching the tunnel barrier in device operation is realized.
Tunnel barrier is commonly a dielectric material and must be ultra thin and extremely uniform in thickness as well as composition. Any inconsistency in terms of chemical stoichiometry or thickness degrades the device performance significantly.
Most typically employed structure of MTJ device is schematically illustrated in FIG. 1, which consists of an antiferromagnetic pinning layer 103, a synthetic antiferromagnetic (SAF) pinned layer 110, a tunnel barrier 107 and a ferromagnetic free layer 108. The synthetic antiferromagnetic (SAF) pinned layer includes a ferromagnetic pinned layer 104, a non-magnetic spacer 105 and a ferromagnetic reference layer 106.
Ever since its discovery, high TMR at room temperature has been one of hot topics of industries due to its spintronics application, such as non-volatile magnetoresistive random access memory (MRAM) and magnetic sensors such as the recording read-head in hard disk drive. For conventional field switching MRAM application, 1 Mbit MRAM with the bit size of 300×600 nm2 requires the MTJ to provide the magnetoresistance (MR) ratio of 40% at the resistance-area (R×A) product of about 1 k-2 k Ωμm2. At the higher density of 250 Mbits, the bit size scales down to 200×400 nm2 and requires MR ratio of higher than 40% at the R×A product of about 0.5 k Ωμm2. Further scaling can be achieved in MRAM by application of magnetization reversal by the spin transfer torque, however, it is required for the MTJ to provide MR ratio higher than 150% at the R×A product range of 10-30 Ωμm2. For the recording read-head in hard disk drive, it is required for the MTJ to provide MR ratio higher than 50% at the R×A product range of 1-2 Ωμm2 in order to pick up reliable signals from the media with areal density of 250 Gbit/in2.
Early efforts made on amorphous AlOx tunnel barrier and ferromagnetic electrodes with high spin polarization were not satisfactory for the requirements mentioned above. Recently single crystal Fe/MgO/Fe has been suggested by theoretical calculation, (Butler et al., Phys. Rev. B 63, (2001) p 054416) and it is predicted that as high as 6000% room temperature-TMR can be obtained due to a superior spin filtering effect of MgO. This spin filtering effect, that is a total reflection of minority spin down electrons in antiparallel magnetization alignment of the two ferromagnetic layers sandwiching MgO tunnel barrier of MTJ, is inherent from the absence of Bloch eigenstates in minority spin-down spin channel with Δ1 symmetry at the Fermi surface. This allows a coherent tunneling, and furthermore enables a giant TMR ratio. There is a microstructural requirement to allow this coherent tunneling, which is the epitaxial growth of Fe (001)/MgO (001)/Fe (001), in that the tunneling electron passes through the (001) atomic planes of Fe and MgO. Experimental attempts to achieve this giant TMR based on single crystal (Fe/MgO/CoFe) growth using molecular beam epitaxy demonstrated room temperature TMR up to 180%. (Yuasa et al. Appl. Phys. Lett. 87 (2005) p 222508) Using MgO tunnel barrier with polycrystalline CoFe ferromagnetic electrodes, 220% room temperature TMR was reported, (Parkin et al. Nat. Mater. 3 (2004) p 862) and even higher TMR reported in MTJ prepared by practical magnetron sputtering on thermally oxidized Si wafer using amorphous CoFeB ferromagnetic electrodes. (Djayaprawira et al. Appl. Phys. Lett. 86 (2005) p 092502)
Great deal of efforts have been made to form the MgO tunnel barrier in the MTJ, which is ultra thin and extremely uniform in thickness as well as composition. Furthermore, similar amount of efforts have been exerted to achieve the crystallinity of MgO tunnel barrier with (001) out-of-plane texture in order to satisfy the microstructural requirement, (001) out-of-plane epitaxy together with bcc-structured sandwiching ferromagnetic layers, given by the theoretical calculation and confirmed by microstructural and thin film chemistry studies. (Y. S. Choi et al. Appl. Phys. Lett. 90 (2007) p 012505, Y. S. Choi et al. J. Appl. Phys. 101 (2007) p 013907)
In general method of preparing MTJ devices for the mass production of MRAM or recording read-head, the deposition of MgO tunnel barrier is divided into the direct deposition and the metal deposition followed by oxidation process. Deposition of tunnel barrier using ceramic target by rf-sputtering or reactive sputtering of metal target in the ambience of gas mixture of oxygen and inert gas falls into the first group of direct deposition. Metal deposition followed by various kinds of oxidation processes, such as natural oxidation, plasma oxidation, radical oxidation or ozone oxidation, falls into the second group.
One of critical bottlenecks for MTJ development is the uniform thickness control of tunnel barrier at the extremely thin thickness. If the thickness of the tunnel barrier is too thin, it is highly possible to contain pinholes, where leak current passes through without spin-dependent tunneling. This degrades signal to noise ratio (S/N) significantly. Another bottleneck is chemical inhomogeneity of tunnel barrier, result in over- or under-oxidation, and the oxidation of underlying ferromagnetic layer. These lead to asymmetric electrical properties with respect to signs of applied bias and abnormal increase of R×A product and decrease of TMR ratio due to the additional tunnel barrier thickness with spin scattering in the surface-oxidized underlying ferromagnetic layer. (Park et al. J. Magn. Magn. Mat., 226-230 (2001) p 926)
Besides the issues of the uniform thickness control of ultra-thin MgO tunnel barrier and the chemical homogeneity across the MgO tunnel barrier, most imminent issue to achieve the giant TMR ratio with low R×A product of MgO-based MTJ is the (001) out-of-plane texture of the ferromagnetic reference layer, MgO tunnel barrier and the ferromagnetic free layer, and the high crystallinity of MgO tunnel barrier. FIG. 2 shows the relationship of MgO texture and crystallinity and the magnetotransport property in CoFeB/MgO/CoFeB MTJ, where MgO is deposited by rf sputtering. It is clearly shown in the FIG. 2A and FIG. 2B that the MTJ prepared with highly crystalline and (001) textured MgO tunnel barrier induces the corresponding (001) texture of CoFe through crystallization of CoFeB amorphous layers by annealing, thus overall (001) texture of CoFeB/MgO/CoFeB is realized. Therefore, it is possible to obtain significantly enhanced MR ratio at low R×A product, as shown in FIG. 2C. However, MTJ with MgO tunnel barrier with poor crystallinity shows very low MR ratio with extremely high R×A product, as also seen in FIG. 2C.
Despite MgO tunnel barrier prepared by rf sputtering has shown great advances by process optimization, there are serious issues, which are hard to overcome for the mass-production, in that MR ratio and R×A product change very sensitively depending on the chamber condition and particle generation inherent from rf-sputtering. (Oh et al. IEEE Trans. Magn., 42 (2006) p 2642) Furthermore, it has been reported that the final R×A product uniformity (1σ) of MTJ devices with MgO tunnel barrier prepared by rf-sputtering is more than 10%, whereas that of MgO tunnel barrier prepared by Mg deposition followed by oxidation process is less than 3%. (Zhao et al. US Patent Application, US 2007/0111332)
Alternative methods of MgO tunnel barrier preparation are the metallic Mg deposition followed by the various oxidation processes or reactive Mg sputtering in the ambience of gas mixture of oxygen and inert gas. Plasma oxidation has been employed in the preparation for AlOx tunnel barrier, however, its high reactivity makes it exceptionally difficult to oxidize ultra-thin metal layer, especially very fast oxidation rate of Mg for MgO formation, precisely down to the interface with the underlying ferromagnetic layer. Thus R×A product and MR ratio of 10000 Ωμm2/45% are obtained by plasma oxidation process, (Tehrani et al. IEEE Trans. Magn., 91 (2003) p 703) whereas those of 1000 Ωμm2/30% by ozone oxidation from MTJ with AlOx tunnel barrier. (Park et al. J. Magn. Magn. Mat., 226-230 (2001) p 926)
Therefore, less energetic oxidation processes have been suggested, which are radical oxidation and natural oxidation to form MgO tunnel barrier. Also reactive sputtering of Mg metal target to form MgO tunnel barrier in the ambience of Ar and O2. FIG. 3 shows the magnetotransport property measurement results obtained from MTJs with MgO tunnel barrier prepared by various methods of MgO tunnel barrier deposition. The MTJ structure is identical except the MgO tunnel barrier part, which is bottom layers/PtMn (15)/CoFe (2.5)/Ru (0.9)/CoFeB (3)/MgO (x)/CoFeB (3)/capping layer. Thickness in parenthesis is in nanometer scale. With reference to the MR ratio and R×A product obtained from the MTJ with MgO prepared by rf sputtering, it is clearly shown that the MR ratio of the MTJ with MgO tunnel barrier prepared by oxidation methods and reactive sputtering is significantly lower. At given R×A product of 10 Ωμm2, MTJ with MgO prepared by rf sputtering provides MR ratio of 180%, whereas MgO deposited by radical oxidation method provides 100%, natural oxidation provides 60%, and MgO prepared by reactive sputtering provides 135%.
The microstructure analyses were carried out with high-resolution transmission microscopy (HREM) and x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). As shown in FIG. 4A and FIG. 4B, it is clearly compared that the difference in the magnetotransport property results from the difference in the crystallinity of MgO tunnel barrier and the lack of epitaxy in CoFeB/MgO/CoFeB layers. FIG. 4A and FIG. 4B are cross-section HREM images taken from the MTJs with MgO tunnel barrier prepared by rf-sputtering and radical oxidation, respectively. As reported by Choi et al. in J. Appl. Phys. 101 (2007) p 013907, CoFeB/MgO/CoFeB-based MTJ prepared by rf sputtering satisfies the microstructural requirement given by the theoretical calculations by Butler et al., in that MgO is highly crystalline and in good grain-to-grain epitaxy with CoFe layers. The CoFe layers are crystallized by post-deposition annealing based on the crystalline MgO as a crystallization template, thus the grain-to-grain epitaxy is realized in CoFe/MgO/CoFe layers. However, MgO tunnel barrier prepared by radical oxidation shows the poor crystallinity mixed with amorphous and it is hard to confirm the pseudo-epitaxy at the interface with CoFe layers.
FIG. 4C shows the clear comparison of the MgO crystallinity and texture with respect to its deposition method, rf-sputtering and natural oxidation. Out-of-plane theta-2 theta scan confirms that MgO tunnel barrier deposited on the amorphous CoFeB layer by rf-sputtering is highly crystalline in as-grown state and highly textured with (001) out-of-plane preferred orientation by pronounced MgO (002) peak at 2 theta=42.4°. However, MgO prepared by metal deposition followed by natural oxidation shows no pronounced peak, which indicates that the MgO layer is almost amorphous.
FIG. 4D and FIG. 4E are XPS spectra obtained from the MTJs with MgO tunnel barrier prepared by rf-sputtering and reactive sputtering, respectively. As reported by Choi et al. in Appl. Phys. Lett. 90 (2007) p 012505, it is critical to have the dominant population of oxygen ions in the lattice point of NaCl-structured MgO for the crystallinity of MgO and higher MR ratio of the MTJ and lower R×A product. It is clear, as shown in FIG. 4D, that the population of oxygen ions (whose binding energy is around 531 eV) occupying lattice point of NaCl-structured MgO is very high in the MgO deposited by rf sputtering, however, there is considerable population of impurity oxygen ion (whose binding energy is around 533.3 eV), as shown in FIG. 4B, which is almost a third of that of oxygen ion in the lattice point in the MgO deposited by reactive sputtering. Thus it can be inferred that this high density of impurity oxygen ions in the MgO barrier is related to the poor crystallinity of MgO and is responsible for the poor MR ratio.
In order to achieve good crystallinity of MgO tunnel barrier prepared by oxidation method, crystalline ferromagnetic reference layer, not bi-layer but single layer, has been employed, in that the structure of MTJ is bottom layers/PtMn (15)/CoFe (2.5)/Ru (0.9)/CoFe (3)/MgO (x)/CoFeB (3)/capping layer. As shown in FIG. 5A, MTJ with fully crystalline CoFe single reference layer provides noticeable drop of MR ratio to 35% from 130% by CoFeB amorphous reference layer. And the shape of full hysteresis loop, FIG. 5B, from MTJ with fully crystalline CoFe single reference layer after as-deposition annealing at 360° C. for 2 hrs under 10 kOe magnetic field indicates that the poor or destroyed SAF structure, whereas that of MTJ, as shown in FIG. 5C, with amorphous CoFeB single reference layer after same condition of post-deposition annealing shows clear SAF coupling in the circle mark. Body-centered-cubic CoFe tends to grow (110) atomic planes parallel to the interface with Ru in order for the lattice match with hexagonal-close-packed Ru (0001) basal plane. (110) out-of-plane texture of ferromagnetic reference layer is not preferable for the giant TMR from the theoretical calculation by Butler et al. in MgO-based MTJ. Furthermore, the thermal stability of SAF(CoFeB/Ru/CoFe) is much worse than that of SAF(CoFeB/Ru/CoFeB), thus clearly distinctive magnetization separation between constituent ferromagnetic layers cannot be secured if the MTJ is composed of CoFeB/Ru/CoFe SAF structure after high temperature post-deposition annealing. Thus the crystalline CoFe single reference layer is proven to be not effective to achieve the good crystallinity of MgO tunnel barrier.
Consequently, it can be understood that the poor crystallinity of MgO tunnel barrier deposited by oxidation method or reactive sputtering cannot play a role of crystallization template to crystallize amorphous CoFeB into CoFe at the CoFeB/MgO interface. Thus no grain-to-grain pseudo-epitaxy can be expected in CoFe/MgO/CoFe layers, which results in the poor magnetotransport property.